Add TRANSFER_TOKEN_SALT to env

[davehorsfall]

Warning: Missing transfer.token.salt: Data transfer features have been disabled.
Please set transfer.token.salt in config/admin.js (ex: you can generate one using Node with crypto.randomBytes(16).toString('base64'))
For security reasons, prefer storing the secret in an environment variable and read it in config/admin.js. See https://docs.strapi.io/developer-docs/latest/setup-deployment-guides/configurations/optional/environment.html#configuration-using-environment-variables.
(Use node --trace-warnings ... to show where the warning was created)